Nice to see you creating a sketchbook, here at MP! Good sketches! I completely agree with the suggestion of "Elemento", about the tower! Also, I feel that, the sketch above it, where the structure is placed right in the middle of the frame, with mountain ranges on it's either side, does not feel like a good choice of composition; you might consider, placing the structure more towards either left or right, and make some interesting shapes with the mountain ranges! Will be waiting for more sketches!